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2.7 Gravitational Field Energy
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I have heard this before and still find it surprising, but am willing to accept it if it allows consistency with theories and calculations. But I dont find the demonstration totally convincing. Allan says there is no gravitational field inside the sphere because the force from around the sphere cancels out. But isnt this only true at the centre? Away from the centre isnt there a net force because such a point is closer to one side of the sphere? So if there is a field there before the collapse, as the field contracts it isnt the case that a field appears where there was no field before.Also what happens if the sphere collapses without generating power. The same region of supposedly no gravitational field will after collapse have the same field with out doing any work. All very interesting!
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Late reply, but a good question! Even away from the center there is no gravitational field inside a spherical shell. If the particle is not in the center, but somewhere else, let’s say somewhere to the right of the center. Then you are correct that the force from any small bit of shell on the right side (closer the the particle) pulling the particle to the right is larger than the force from the left side (far away from the particle) pulling the particle back to the center. However because the particle is not in the center, a larger part of the sphere is on the left of the particle pulling the particle back to the center, than on the right of the particle pulling it further away from the center. So, while the force by any small bit of area from the closer side to the sphere is larger, there are more bits of area on the far side of the sphere. These effects cancel out exactly so the net force is 0 anywhere inside the shell.
To your second question, a different way to look at it gravitational energy being negative is through kinetic energy. Take two particles which are not moving so they have no kinetic energy. The gravitational attraction will cause them to move towards each other, so they build up velocity and gain kinetic energy. This kinetic energy has to come from somewhere: the gravitational field. So to conserve total energy, if their kinetic energy increases, the energy of the gravitational field must decrease. The same happens if the sphere collapses without generating power. The energy adding an extra region of gravitational field as mentioned in the talk goes into the motion of the shell. This motion can be used to generate power (only as thought experiment of course)
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I had never before heard of the Gravitational field having negative energy and I was so surprised to see how Prof. Guith through a simple thought experiment established it. And yes I do believe in the derivation as so far we have not encountered any exception to the conservation of energy and this also successfully shows that indeed energy would be conserved during inflation if we agree on the fact that gravitational filed have negative enegry.
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Hello Ladies and Gentlemen,
I have no exposure to negative gravity fields. It does not surprise me, growing up near a nuke power plant allows open minds to science.
I am convinced by the lecture to keep listening to highly decorated Prof. Guth until i have the
Aha!.Most things in existence have a yin and yang, so to hear the gravity field as yeilding, negative, wet, and yin is ok.
It will have its` mirror.
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some of this is like hearing just believe, and then given a number or formula that seems to only satisfy the fitting of the theory being postulated, then if it doesn’t fit observations adding another constant or formula. Or maybe I just don’t get it yet. but I’m still learning, so maybe there’s hope for me in the future.
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